The present invention is concerned generally with the production of brown coal and lignite and more particularly with separating sand from a mined brown coal or lignite material which contains sand.
The terms brown coal and lignite are used interchangeably in this specification, although modern research may indicate that there may be certain differences between brown coal and lignite, from certain points of view; accordingly, where one such term is used, it is also to be deemed to include the other such term.
Although, by virtue of its chemical composition, sand is entirely inert in its behaviour upon combustion of or in other operations involving transforming brown coal or lignite, and therefore does not cause any environmental pollution, it is at least desirable and generally virtually essential for it to be at least substantially removed from the brown coal or lignite, when the amount of sand exceeds a given proportion, as otherwise a considerable amount of wear will occur in boilers or other equipment in which the sand-bearing coal or lignite is for example converted into gas or into liquid or reacted in some other manner.
The occurrence of a certain content of sand in mined brown coal or lignite, which constitutes a nuisance, is frequently inevitable even when the brown coal or lignite beds or seams are free from sand inclusions. However, with the present-day extraction methods and equipment, it is often inevitable that sand-bearing strata are also removed when mining coal from the roof and/or floor of a seam; that sand mixes with the coal and thus results in the raw brown coal or lignite material which comes out of the mine having a certain sand content. The sand content may be so high under some circumstances as to give rise to the difficulties referred to above. A similar situation may also arise for example when a stratum of sand of low thickness or strength occurs between two directly adjoining seams, as in such cases the stratum of sand cannot generally be shored up in the mining operation, and the mined material will inevitably include even higher proportions of sand.
These difficulties, which will be seen to arise due to the very nature of the beds or deposits and/or the mining process, have already been encountered for some decades, so that the problem of separating sand from a mining run of brown coal or lignite is not a new one. Accordingly, various attempts have been made in the past to overcome this difficulty and to remove sand from mined brown coal or lignite, or at least reduce the proportion of sand in such material to such an extent that the sand remaining in the brown coal or lignite does not give rise to difficulties such as those outlined above when the brown coal or lignite is subsequently put to use, and in particular therefore does not cause a significant amount of wear.
The reason that the problem of separating sand from a sand-bearing brown coal or lignite mix has not hitherto found a generally applicable solution is more particularly that the expenditure involved in separating out the sand cannot be at a very high level, as otherwise it becomes uneconomical to use the brown coal or lignite. This is essentially because the heating value of the brown coal or lignite is low, at any rate in comparison with other fossile fuels.